The invention relates to a control apparatus that utilizes solenoid valves for controlling hydraulic servo units which are used, for example, for speed reduction ratio control and clutch control required in a variable speed transmission.
There have been proposed various types of continuously variable speed transmissions for vehicles used in continuously varying the input rotational speed and transmitting it as an output. For example, Japanese Patent Laid-Open Publication No. 56(1981)-95722 discloses a continuously variable speed transmission for vehicles, comprising a closed hydraulic circuit having therein a constant displacement hydraulic pump and a variable displacement hydraulic motor.
In such a continuously variable speed transmission, clutch operation at the time of starting or stopping, and speed reduction ratio change during cruising, are controlled by servo units based on signals representing the engine throttle opening and the vehicle speed. For example, such control may be given as shown in FIG. 7A, by means of four solenoid valves 163a through 163b which serves to increase or decrease the hydraulic pressure Pl of the working fluid supplied from the line 162 connected with the right and left cylinders of the servo cylinder 161, or as shown in FIG. 7B, by means of two solenoid valves 166a and 166b which function via a four-way valve 165.
In the case of FIG. 7A, however, four solenoid valves are needed, and in the case of FIG. 7B, a four-way valve is needed, though four valves are not required. In any case, the use of these valves makes the design, the manufacture, and the control of the servo unit not only complicated but also costly. In particular, in a continuously variable speed transmission this leads to a disadvantage in that the transmission becomes large in size and complex, since the transmission requires servo units for controlling speed reduction ratio and clutch operation, respectively.
Another disadvantage with a conventional servo unit is that it cannot provide desirable control in the case of solenoid valve malfunction. Therefore, various designs for such servo units have been disclosed in the prior art as containing fail-safe measures in the event of such solenoid valve malfunction, in which, for example, one of the servo units is slowly operated by slowly releasing the clutch off. For example, Japanese Patent Laid-Open Publication No. 60(1985)-249761, discloses an apparatus in which a four-way valve undertakes pressure control in the right and left cylinder chambers of the servo cylinder for ratio control of a continuously variable speed transmission. The operation of the four-way valve being controlled by a solenoid valve and, in case the solenoid valve is inoperable due to its malfunction, either the four-way valve is maintained so as to prohibit the ratio control or the hydraulic fluid is furnished through an orifice so as to slowly vary the reduction ratio until the ratio reaches its maximum or minimum.
However, if the reduction ratio is prohibited to change as illustrated above, it remains so and cannot be reduced to LOW (maximum speed reduction ratio) when the vehicle is started again, and hence a smooth start will not be available. Furthermore, the use of a four-way valve has above-mentioned disadvantage that the control apparatus becomes large in size and complex.
Clutch control for a transmission is, as mentioned above, often given by means of servo units utilizing solenoid valves. With these systems it may happen that the solenoid valves fail to operate, leaving the clutch in operational engagement even if the shift lever is brought back to its neutral position and an OFF signal is generated.